Implantable cardiac stimulation devices are currently being used to treat various types of arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF). Such devices are capable of detecting the occurrence of an arrhythmia, and automatically applying an appropriate electrical stimulation or shock therapy to the heart aimed at terminating the detected arrhythmia. In addition to providing automatic stimulation, such devices often include circuitry that is configured to acquire intracardiac electrogram (IEGM) signals, convert the raw analog data into digital signals, and store the digital signals for later processing and/or telemetric transmission to an external device.
Recently there has been increased interest in adding monitoring capabilities to implantable cardiac devices. For example, many publications have suggested attaching various types of implantable sensors to the housings of pacemakers and/or implantable cardioverter-defibrillators (ICDs), or adding such sensors to leads that attach to pacemakers and/or ICDs. Examples of such sensors include implantable venous oxygen saturation sensors, including implantable pulse oximetry sensors, which include implantable light sources and light detectors that can be used to monitor blood oxygen concentration levels. While such optical sensors are useful, they may add to the cost and complexity of implantable cardiac systems. Additionally, such optical sensors may cause an increased drain on device batteries, potentially reducing the longevity of such devices. Accordingly, it would be useful if alternative and potentially more efficient techniques for monitoring venous oxygen saturation were available. Preferably such techniques would require minimum changes to existing pacemakers and ICDs.